The most commonly used technique at the present time for determining fiber volume for graphite-epoxy composite materials is the acid digestive method, which is described in reference manual ANSI/ASTM-D-3171-76 published by the American Society for Testing Materials. The acid digestive method is an inherently destructive testing method.
Ply count for a graphite-epoxy composite material is generally determined at the present time by sectioning of the composite material. Sectioning is, of course, an inherently destructive testing method.
Pulsed eddy current techniques for nondestructive testing are discussed in texts such as:
a. Introduction to Electromagnetic Nondestructive Test Methods by H. L. Libby, published by R. E. Krieger Publishing Co. (1979), pages 122-177 and 258-268; and
b. Nondestructive Testing by W. J. McGonnagle, published by Gordon and Breach (1977), pages 382-385.
However, until the present invention, no eddy current technique had been developed that was capable of uniquely determining fiber volume, ply count or thickness of any fiber-containing composite material.